The present invention relates to rear projection screens chiefly used for rear-projection-type projectors such as video projectors and slide projectors.
As rear projection screens of this type, there have conventionally been known those screens composed of a single lenticular lens sheet comprising as a base material a synthetic resin such as polymethyl methacrylate, and those screens composed of such a lenticular lens sheet and other lens sheets. To form images on these rear projection screens, imaging light is projected on the screens by using light sources such as CRTs.
In recent years, projection tubes having small projection apertures, such as liquid crystal projectors and light bulbs came to be used as light sources in place of CRTs. However, the conventional rear projection screens have such a problem that, when images are formed on these screens by the use of projection tubes having small projection apertures, scintillation or speckle is caused on the images.
In order to solve this problem, there have conventionally been proposed a method in which the screens are scanned by using laser light sources (see Japanese Patent Laid-Open Publication No. 173094/1993); a method in which the screens are vibrated (see Reference 1 (J. Opt. Soc. Am., Vol. 66, No. 11, Nov. 1976, xe2x80x9cSpeckle-free rear-projection screen using two close screens in slow relative motionxe2x80x9d)); and a method in which large amounts of diffusers are incorporated into lens sheets.
However, in the aforementioned conventional methods, the modification of the projectors themselves is needed, or additional apparatus are required in order to prevent images from undergoing scintillation or the like. Further, the incorporation of large amounts of diffusers into lens sheets causes such troubles that the gain is decreased and that the resolution is unfavorably lowered.
The present invention was accomplished in view of the above-described drawbacks. An object of the present invention is therefore to provide a rear projection screen capable of forming thereon an image free from scintillation or the like with the decrease in gain and resolution minimized without using any additional apparatus even when a projection tube having a small projection aperture is used.
The first aspect of the present invention is a rear projection screen comprising a lens sheet having an optical function of condensing or diffusing light, wherein the lens sheet has two or more diffusing parts separately provided in a light-transmitting direction.
In the first aspect of the present invention, it is preferable that one of the two or more diffusing parts be provided on a light-entering-side surface of the lens sheet and that another one of the diffusing parts be provided on a light-emerging-side surface of the lens sheet. Further, it is preferable that the two or more diffusing parts be provided on a surface of the lens sheet and inside the same.
The second aspect of the present invention is a rear projection screen comprising two or more lens sheets or optical sheets having an optical function of condensing or diffusing light, wherein at least one of the two or more lens sheets or optical sheets has at least one diffusing part, and the two or more lens sheets or optical sheets have, as a whole, two or more diffusing parts.
In the second aspect of the present invention, it is preferable that the diffusing parts be provided on surfaces of the two or more lens sheets or optical sheets, or inside the same. Further, it is preferable that the diffusing part of the outermost lens sheet or optical sheet on a light source side be provided on the light-entering-side surface of this lens sheet or optical sheet and that the diffusing part of the outermost lens sheet or optical sheet on an observation side be provided on a light-emerging-side surface of this lens sheet or optical sheet.
In the above-described first and second aspects of the present invention, any two of the two or more diffusing parts are preferably such that a light-source-side diffusing part has a diffusing power lower than that of an observation-side diffusing part. Further, any two of the two or more diffusing parts are preferably such that a light-source-side diffusing part is formed by incorporating first diffusive fine particles into a first base material, that an observation-side diffusing part is formed by incorporating second diffusive fine particles into a second base material and that a difference between a refractive index of the first diffusive fine particles and that of the first base material is smaller than a difference between a refractive index of the second diffusive fine particles and that of the second base material.